Linear induction motors are the preferred choice in transportation systems due their simplicity, price, and reliability. They are utilized as means of propulsion and in some cases as suspension, as well as guidance.
These are usually two physically separate systems built and controlled independently. One set of linear motors produces propulsion forces while the totally independent system generates magnetic levitation (suspension) forces, recognized as MAGLEV. The known MAGLEV systems are typically based on magnetic attraction between two ferromagnetic parts. However, some MAGLEV systems are based on magnetic repulsion and they are typically air-cored.
The TransRapid railing system has been developed in Germany, which utilizes electromagnetic suspension based on attraction between on-board electromagnets and iron-backed guideway rails. The system is expensive, not very efficient, and not stable at very high speeds (He et al., 1992).
The MLX railing system, developed in Japan, utilizes electrodynamic suspension (EDS) to levitate the train. In this railing system, magnets on the train induce currents in the guiding rails. These currents create magnetic fields which interact with the original field of the magnets. Levitation is supported by the repulsive force between the two fields. The magnets on the train are either electromagnets or an array of permanent magnets. The advantage of EDS systems is that they are naturally stable at high speeds and thus no feedback control is needed, unlike the TransRapid System. However, EDS systems have disadvantages: they are very expensive and the train must be equipped with wheels because at slow speeds the induced currents are not strong enough to support levitation (Hikasa and Takeuchi, 1980).
The Inductrack method, developed in the United States, utilizes passive levitation. No external power is needed to levitate the train. The levitation is produced by the motion of on-board permanent magnets over the rail so that induced eddy currents obtain the repulsion force (Friend, 2004).
Hyperloop Transportation Technologies and Hyperloop One utilize very similar suspension methods as the Inductrack. The levitation is produced by the motion of an on-board Halbach array of permanent magnets over the conductive rail. Among others, the system has one major disadvantage: the passive suspension typically is not controllable (Bambrogan at al., 2017, and other Hyperloop One patents).
Many attempts have been made to combine these two functions (propulsion and suspension) in one system, but based on practical linear induction motors. It is known that linear induction motors can produce normal forces (lift) together with thrust, but the problem is producing a successful control method of a combined propulsion-suspension drive system.
A few attempts have been made to create such a control system:                Morizone at al. propose the novel control method of the thrust force and the attractive force of LIM driven by the power source with the frequency component synchronous with the motor speed. In this control method, it is possible to apply the conventional vector control to the thrust force control of LIM. The frequency component synchronous with the motor speed does not generate the thrust force. Therefore the frequency component synchronous with the motor speed is able to adjust the attractive force for levitation without influence on thrust force control (Morizane et al, 2011).        Verdel's paper presents a feasibility study of a novel magnetic levitation system through the use of linear induction motor (LIM) segments implemented in a rotating ring system.        It investigates the best manner of simultaneous and decoupled control of thrust and normal forces generated by the single-sided LIM (SLIM) (Verdel, 2007).        In 2018, Korean scientists proposed an all-in-one system for a hyperloop that conducts propulsion, levitation, and guidance. It is based on a non-symmetric double-sided linear induction motor (Ji et al, 2018).        Cox et al present a new method of combined electromagnetic levitation and propulsion using a double-sided linear induction machine and a simple conductive sheet secondary. If the supply phase angle of one primary is modified with respect to that of the other, a controllable lift force can be developed on the conductive secondary and its load at any velocity or when stationary (Cox et al, 2016).        Japanese scientists propose a decoupled-control method of normal and thrust forces in single-sided linear induction motor (SLIM) which is based on a unified concept of machine principle. This method is derived from the analytical formulas for normal and thrust forces of a SLIM. This method can be applied to a LIM vehicle system, in which the normal force is used to levitate and the thrust force to propel a LIM vehicle without force-couple of LIM. By using this method, a compact combined levitation-and-propulsion system with LIM only can be realized. This method is verified by a successful simulation of levitation and propulsion for a SLIM model vehicle (Kinjiro et al, 2000, and Kinjiro, 1991).        
The objective of this invention is to resolve the problem of controlling a system of linear induction motors, which can be grouped so as to produce both propulsion and levitation simultaneously but which are both independently controllable.